PAKSONG, Laos (NYTimes) - Petchinda Chantamart first heard what sounded like a bomb going off a few miles away. Then came a curious noise, like a strong wind.

She knew instinctively what it meant: One of the new dams under construction near her village in southern Laos had failed. She began banging on her neighbours' doors, she recounted, urging them to flee to higher ground.

"The water is coming!" Chantamart roared.

Within a half-hour, the water in her village, Xay Done Khong, was more than 30 feet deep and rising.

Chantamart, 35, and many of her neighbours escaped the deadly flood. But others were not so lucky when an auxiliary dam, part of the billion-dollar Xe-Pian Xe-Namnoy hydroelectric project, failed on Monday (July 23) evening amid heavy rains, sending more than 170 billion cubic feet of water rushing downstream.

At a news conference on Wednesday (July 25), Prime Minister Thongloun Sisoulith said that 131 people were still missing and more than 3,000 were homeless. Many had been rescued from rooftops and trees after villages and farmland were flooded.

At least 26 people have been reported killed. More than 130 were reported missing.

"A second step for us will be to recover and identify the deceased, but for now, we hurry to find those who are still alive in the area," Bounhom Phommasane, the governor of the district of Sanamxay, told The Vientiane Times.

Chantamart said that hundreds of people from her village had escaped, but that 15 people were still missing, nine of them children. She had been unable to reach their homes on Monday because the floodwaters had climbed too high.

"I'm very worried about them, from the bottom of my heart," she said.

After she and hundreds of others scrambled to higher ground on Monday, soldiers and local officials moved them to the town of Paksong, west of the dam site, to take refuge in an empty warehouse normally used to store coffee.

Video posted by the Thai News Agency showed vast quantities of water cascading over what appeared to be the diminished structure of the dam, known as Saddle Dam D.

The official Lao News Agency reported that the dam had collapsed. The main builder of the hydropower project, SK Engineering & Construction of South Korea, said it would investigate whether the dam had collapsed or overflowed because of heavy rains.

International Rivers, an advocacy group that has opposed the rapid growth of hydropower dams in Laos, said in a statement posted online that the auxiliary dam had collapsed as flooding from heavy monsoon rains caused it to overflow on Monday night.

The group, which seeks to protect rivers around the world, said the disaster showed that many dams were not designed to handle extreme weather events like the rains on Monday.

"Unpredictable and extreme weather events are becoming more frequent due to climate change, posing grave safety concerns to millions who live downstream of dams," International Rivers said.

People living below the dam had only a few hours' warning to evacuate before it failed, according to the group.

"Communities were not given sufficient advanced warning to ensure their safety and that of their families," the statement said.

"This event raises major questions about dam standards and dam safety in Laos, including their appropriateness to deal with weather conditions and risks."

Seven villages in Sanamxay, which is in Attapeu province, were flooded and more than 6,000 people were displaced by the dam's failure, officials said.

The disaster cleanup may be further complicated by old U.S. bombs and other explosives buried in the area, a legacy of the Vietnam War era that has haunted Laos for decades.

Attapeu province, which borders Vietnam and Cambodia, is heavily contaminated with what disarmament experts call unexploded ordnance, which can detonate on unsuspecting civilians even after decades of lurking undisturbed.

The flooding could make the ordnance harder for decontamination teams to find.

"There is immediate concern for the safety of personnel from survey, clearance and survivor assistance programs who may have been in the impacted area at the time," said Mark Hiznay, associate arms director at Human Rights Watch's Washington office.

The Xe-Pian Xe-Namnoy is one of 70 hydropower plants that are planned, underway or have been built in Laos, most of them owned and operated by private companies, International Rivers said.

The project consists of major dams on three tributaries of the giant Mekong River as well as several smaller auxiliary dams, or saddle dams, including the one that failed.

South Korea and Thailand were mobilising emergency assistance. Companies from both countries are involved in building and financing the Xe-Pian Xe-Namnoy project, which was supposed to provide 90 percent of its electricity to Thailand once it began operating.

President Moon Jae-in of South Korea instructed his government on Wednesday to dispatch a rescue and emergency relief team to Laos.

"The investigation is still underway to find out the causes of the dam incident, but our government should waste no time in actively participating in the rescue and relief operations at the scene because our own businesses are involved in the construction," Moon said, according to his office.

Repeated phone calls to the spokesman's office at SK Engineering & Construction's headquarters in Seoul went unanswered on Wednesday.

Korea Western Power Co., which has a contract to operate the power plant when it is completed, said its officials and workers from SK Engineering & Construction in Laos had joined the rescue and relief efforts.

SK deployed one helicopter and 11 boats, and Korea Western Power sent two boats and its local medical staff.

On Wednesday evening, a heavy rain was falling on the corrugated roof of the makeshift shelter in Paksong where a few hundred people had found shelter.

The sky, cloudy in the afternoon, had turned as murky as gauze. A few ambulances streaked by in the gathering dusk, leaving smudgy trails of red and blue lights in their wake.

Inside, adults and children were milling around in sandals and soiled clothes, eating sticky rice from plastic foam bowls. Some sat on blue-and-orange tarps that had been spread on the concrete floor, and many looked on with vacant stares.

A makeshift canteen, with steaming pots of sticky rice, had been set up in the warehouse's covered parking lot.

Chantamart said she had little hope that anything was left of her home or her village.

"Every single house, gone," she said.

Chantamart said she was not sure whom to hold responsible for the flood. But she said the government and the company behind the dam should take more action to help the victims.

"People here are shocked, scared and sorry for each other because of our loss," she said, as children in soiled sweatshirts crowded around her.

About 70 per cent of the people in her village were from minority ethnic groups, she said. Most grew rice and coffee. Occasionally, they found work as day labourers.

Khamla Souvannasy, an official from Paksong, said local authorities were struggling to support the hundreds of people who had gathered at the warehouse.

"The weather is an obstacle," he said, as a bout of particularly heavy rain lashed the warehouse's roof.

"We're still looking for mattresses."

He added, "The disaster came so quickly. There's no way to be prepared for that, but we'll just keep working and working."

"Everyone here lost everything - animals, our houses," said Den Even Den, a farmer from Xay Done Khong. "All we have left is our lives."